WAUKESHA COUNTY -- A jury has been selected in the trial of Anissa Weier, one of two girls accused in the so-called Slenderman stabbing in Waukesha County.

Sixteen jurors will hear the testimony in this case. Four will be alternates. Once the testimony is complete, 12 of the 16 jurors will decide the fate of Weier.

Anissa Weier

Prosecutors say Anissa Weier and Morgan Geyser lured their friend into a Waukesha park and stabbed her 19 times in 2014. All three girls were 12 years old at the time. Weier and Geyser told police they committed the act to appease the fictional character Slenderman.

Weier pleaded guilty in August to attempted second-degree homicide, but maintains she's not responsible due to a mental disease or defect.

Attorneys on both sides selected jurors on Monday to decide whether she was suffering from a mental defect. A plea agreement calls for Weier to spend at least three years in a mental hospital if she's found to have been mentally ill, and 10 years in prison if she's not.

Anissa Weier

All jurors will be sequestered in this case. They are packing their bags on Monday -- and must return to the Waukesha County Courthouse at 8:00 p.m. where they will be taken to a hotel.

Prosecutors allege that Weier and her friend, Morgan Geyser, lured a classmate into a Waukesha park in May 2014 and stabbed her 19 times. The girls have said it was an effort to please Slenderman and become his servants, or to keep the character from attacking their families. All three girls were 12 years old at the time. Weier and Geyser left their classmate for dead and starting walking to the Nicolet National Forest, where they hoped to join Slenderman in his mansion. A passing bicyclist found the classmate, who survived, and Weier and Geyser were captured later that day.

Slenderman

WHAT ARE THE CHARGES?

Anissa Weier

Prosecutors charged both girls with being a party to attempted first-degree intentional homicide. Weier struck a deal with prosecutors in August in which she pleaded guilty to being a party to attempted second-degree intentional homicide, essentially acknowledging she committed all the elements of the offense. But she also has pleaded not guilty due to mental illness, meaning she believes she isn't responsible for her actions. The jury will decide whether she was indeed impaired.

WHAT DOES THE EVIDENCE REVEAL ABOUT WEIER'S MENTAL STATE?

Weier told a judge during her plea hearing in August that she believed Slenderman would attack her and her family if she didn't kill the classmate. Psychologists have testified that she suffered from persistent depression and a delusional disorder linked to schizotypy, a diminished ability to separate reality from fantasy. It's unclear what evidence Weier might present. Her attorney, Maura McMahon, didn't return a voicemail. Waukesha County District Attorney Susan Opper declined to comment on evidence or strategy.

Judge Michael Bohren

HOW WILL THE TRIAL WORK?

Waukesha County Judge Michael Bohren has set aside up to two weeks for the trial. A valid verdict needs at least 10 of the 12 jurors to agree. The defense has the burden of proving Weier was mentally ill at the time of the attack and whether, as a result of that illness, she couldn't appreciate how wrong her actions were or follow the law.

She has pleaded not guilty to being a party to first-degree attempted homicide. Her trial is set to begin Oct. 9.

Morgan Geyser

WHAT DO EXPERTS THINK ABOUT THIS CASE?

In Wisconsin, anyone 10 or older charged with attempted first-degree or second-degree intentional homicide is considered an adult. Cecelia Klingele, a University of Wisconsin law professor who researches sentencing policy and the consequences of convictions, calls the case a tragedy for everyone involved. The victim suffered serious injury and the other girls clearly needed guidance that they didn't get, she said. But trying to punish children through a system designed for full-grown adults doesn't make any sense, since in almost all other circumstances, the state recognizes that children are not as responsible as adults for their actions.